Niels Krebs

Plasma Surface Modification of Glass-Fibre-Reinforced Polyester Enhanced by Ultrasonic Irradiation

During atmospheric pressure plasma treatment, reactive species generated in the plasma diffuse through a boundary gas layer which is adsorbed at the material surface. Many of the reactive species become inactivated before reaching the surface due to their short lifetime. The efficiency of plasma treatment can be highly enhanced by simultaneous high-power ultrasonic irradiation of the treating surface, because the delivered acoustic energy can reduce the thickness of the boundary gas layer. Here surfaces of glass-fibre-reinforced polyester (GFRP) plates were treated using an atmospheric pressure dielectric barrier discharge in helium with ultrasonic irradiation, particularly for the adhesion improvement. The ultrasound was irradiated through a powered mesh electrode using a high-power gas-jet ultrasonic generator. The discharge mode changed from glow to filamentary by the ultrasonic irradiation. The surface characterizations were performed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force mictroscopy (AFM). O/C ratios at the GFRP surfaces before the treatments, after 30-s plasma treatment, and after 30-s plasma treatment with ultrasonic irradiation were 0.295, 0.385 and 0.447, respectively. This indicated that the plasma treatment oxidized and roughened the GFRP surface, and the ultrasonic irradiation further enhanced the oxidation. It is concluded that plasma treatment efficiency for adhesion improvement of GFRPs is enhanced by the ultrasonic irradiation.

Ultrasound enhanced plasma surface modification at atmospheric pressure

Abstract Efficiency of atmospheric pressure plasma treatment can be highly enhanced by simultaneous high power ultrasonic irradiation onto the treating surface. It is because ultrasonic waves with a sound pressure level (SPL) above ∼140 dB can reduce the thickness of a boundary gas layer between the plasma and the material surface, and thus, many reactive species generated in the plasma can reach the surface before they are inactivated and can be efficiently utilised for surface modification. In the present work, glass fibre reinforced polyester plates were treated using a dielectric barrier discharge and a gliding arc at atmospheric pressure to study adhesion improvement. The effect of ultrasonic irradiation with the frequency diapason between 20 and 40 kHz at the SPL of ∼150 dB was investigated. After the plasma treatment without ultrasonic irradiation, the wettability was significantly improved. The ultrasonic irradiation during the plasma treatment consistently enhanced the treatment efficiency. The principal effect of ultrasonic irradiation can be attributed to enhancing surface oxidation during plasma treatment. In addition, ultrasonic irradiation can suppress arcing, and the uniformity of the treatment can be improved.

Ultrasound Enhanced Plasma Treatment of Glass-Fibre-Reinforced Polyester in Atmospheric Pressure Air for Adhesion Improvement

A glass-fibre-reinforced polyester (GFRP) plate was treated with dielectric barrier discharge (DBD) at atmospheric pressure in air for adhesion improvement. The effects of ultrasonic irradiation using a high-power gas-jet generator during the treatment were investigated. The optical emission spectrum of the discharge remained almost unchanged by the ultrasonic irradiation, indicating that the bulk property of the discharge was not significantly influenced by the ultrasound. However, the ultrasonic irradiation during the plasma treatment suppressed occasional arcing in the DBD, preventing damage of the GFRP plates. The polar component of the surface energy of the polyester plate was 21 mJ/m2 before the treatment, increased markedly to 52 mJ/m2 after 2-s plasma treatment without ultrasonic irradiation, and further increased slightly after longer treatments. In addition, the polar component of the surface energy increased due to the simultaneous ultrasonic irradiation, indicating that the adhesive property would be further improved. This result shows a good agreement with surface characterization by X-ray photoelectron spectroscopy. Time-of-flight secondary ion mass spectrometry ion images show that nitrogen-containing functional groups were uniformly attached after the treatments. The roughness of the GFRP surfaces increased after the plasma treatment, but the ultrasonic irradiation did not enhance surface roughening.

Ozone Production in a Dielectric Barrier Discharge with Ultrasonic Irradiation

Ozone production has been investigated using an atmospheric pressure dielectric barrier discharge in pure O2 at room temperature with and without ultrasonic irradiation. It was driven at a frequency of either 15 kHz or ∼40 kHz. The ozone production was highly dependent on the O2 flow rate and the discharge power. Furthermore, powerful ultrasonic irradiation at a fundamental frequency of ∼30 kHz with the sound pressure level of ∼150 dB into the discharge can improve the ozone production efficiency, particularly when operated at the frequency of 15 kHz at the flow rate of 15 L/min.

Ultrasound enhanced 50 Hz plasma treatment of glass-fiber-reinforced polyester at atmospheric pressure

Glass-fiber-reinforced polyester (GFRP) plates are treated using a 50 Hz dielectric barrier discharge at a peak-to-peak voltage of 30 kV in helium at atmospheric pressure with and without ultrasonic irradiation to study adhesion improvement. The ultrasonic waves at the fundamental frequency of around 30 kHz with the sound pressure level of approximately 155 dB were introduced vertically to the GFRP surface through a cylindrical waveguide. The polar component of the surface energy was almost unchanged after the plasma treatment without ultrasonic irradiation, but drastically increased approximately from 20 up to 80 mJ m−2 with ultrasonic irradiation. The plasma treatment with ultrasonic irradiation also introduced oxygen- and nitrogen-containing functional groups at the GFRP surface. These changes would improve the adhesion properties of the GFRP plates.

Observation of gliding arc surface treatment

Abstract An alternating current (AC) gliding arc can be conveniently operated at atmospheric pressure and efficiently elongated into the ambient air by an air flow and thus is useful for surface modification. A high speed camera was used to capture dynamics of the AC gliding arc in the presence of polymer surfaces. A gap was observed between the polymer surface and the luminous region of the plasma column, indicating the existence of a gas boundary layer. The thickness of the gas boundary layer is smaller at higher gas flow-rates or with ultrasonic irradiation to the AC gliding arc and the polymer surface. Water contact angle measurements indicate that the treatment uniformity improves significantly when the AC gliding arc is tilted to the polymer surface. Thickness reduction of the gas boundary layer, explaining the improvement of surface treatment, by the ultrasonic irradiation was directly observed for the first time.

Influence of ultrasonic irradiation on ozone generation in a dielectric barrier discharge

An atmospheric pressure dielectric barrier discharge (DBD) was generated in an N2/O2 gas mixture at room temperature with and without ultrasonic irradiation to investigate ozone production. Powerful ultrasonic irradiation with the sound pressure level of approximately 150 dB into the DBD can enhance ozone production especially when the DBD was driven at a frequency of 15 kHz.

Plasma treatment of carbon fibres and glass-fibre-reinforced polyesters at atmospheric pressure for adhesion improvement

Atmospheric pressure plasma treatment is useful for adhesion improvement, because cleaning, roughening and addition of polar functional groups can be expected at the surfaces. Its possible applications in the wind energy industry include plasma treatment of fibres and fibre-reinforced polymer composites before assembling them to build wind turbine blades. In the present work, unsized carbon fibres are continuously treated using a dielectric barrier discharge plasma in helium at atmospheric pressure, and carbon fibre reinforced epoxy composite plates are manufactured for the mechanical test. The plasma treatment improved fracture toughness, indicating that adhesion between the fibres and the epoxy was enhanced by the treatment. In addition, glass-fibre-reinforced polyester plates are treated using a gliding arc and an ultrasound enhanced dielectric barrier discharge, improving the wettability and/or the adhesive strength with a vinylester resin.